PROJECT SUMMARY/ABSTRACT Huntington's disease (HD) is a fatal neurodegenerative disorder affecting the physical, mental, and emotional state of approximately 1 in 10,000 individuals. There is no cure or effective treatment for this disease, a factor that stems directly from the lack of knowledge regarding the mechanism underlying this disorder. HD is caused by a genetic mutation in the polyglutamine (polyQ) domain of Huntingtin exon 1 (Htt_ex1). This mutation causes the polyQ tract to become pathologically expanded (>36Q), and for reasons that remain unknown, this expansion alters the function of the protein and causes it to become toxic and prone to misfolding and aggregation. In order to understand how a polyQ expansion leads to disease, I will utilize a combination of spectroscopic techniques to identify and characterize the molecular features that are affected in huntingtin (Htt) monomers and fibrils. Monomers and fibrils represent important conformers in the aggregation pathway and are prominent sources of toxicity. Specifically, I aim to investigate how the C-terminal domain of these conformers is affected by a polyQ expansion and whether this domain (C-terminus) facilitates the interaction of Htt and the chaperone DnaJB1. Mounting evidence, including recent findings from our lab, has alluded to the importance of this region in the overall organization of toxic Htt fibril species. Characterization of this region may therefore hold the key to understanding the mechanism of Htt aggregation and reveal potential targets for the disaggregation of fibrils. In aim 1, I will characterize the structure of mutant (>36Q) and wild type (<35Q) monomers using solid-state Nuclear Magnetic Resonance (ssNMR) and an innovative sample preparation method that allows for the trapping of Htt in its monomeric state. In doing so, I plan to uncover conformational changes that contribute to the pathogenesis of HD. In aim 2, I will utilize ssNMR and Electron Paramagnetic Resonance (EPR) to identify the sites that facilitate Htt fibril recognition by the chaperone DnaJB1. Identification of such sites will be key in understanding the mechanism underlying aggregate identification and disaggregation. Ultimately, my findings will allow for greater insight into the molecular features driving the mechanism of protein misfolding, disaggregation, and toxicity in Huntington's disease, and thereby provide key targets for the development of efficacious therapeutics.